Negative working thermal sensitive printing plate precursors comprising thermoplastic polymer particles, of which the imaging mechanism is based on coalescence or coagulation of the polymer particles, have been disclosed in several applications. For example EP-As 770 494, 770 495, 770 496 and 770 497 disclose printing plate precursors comprising thermoplastic particles which are, after exposure, processed on press by supplying ink and/or fountain solution.
EP-A 1 342 568 and WO2006/037716 disclose a method of making a lithographic printing plate wherein a precursor comprising thermoplastic particles is, after exposure, processed in a gum solution. EP-As 1 614 539 and 1 614 540 disclose a method of making a lithographic printing plate wherein a precursor comprising thermoplastic particles is, after exposure, processed in an alkaline solution.
EP-As 1 736 312 and 1 910 082 disclose lithographic printing plate precursors comprising thermoplastic particles and an infrared dye, the infrared dye being capable of forming a print-out image upon exposure to infrared radiation.
EP-As 1 859 935 and 1 859 936 disclose a lithographic printing plate precursor comprising thermoplastic particles having an average particle size between 10 and 40 nm and wherein the amount of the infrared dye is adjusted as function of the particle size of the polymer particles.
WO 2010/031758 discloses a lithographic printing plate precursor comprising thermoplastic polymer particles and an infrared dye characterized in that the dye contains a substituent selected from bromo and iodo.
It has been observed that a disadvantage of precursors comprising well known infrared dyes may be a poor stability when stored under daylight conditions. It seems that the infrared dyes may be subjective to oxidative and/or light induced deterioration when the precursors are stored under daylight conditions. This instability may result in a loss of sensitivity, a worsening of the clean-out behaviour, i.e. removal of the non-image areas during development, and a change of colour of the precursor. These phenomena become more pronounced under prolonged storage under daylight conditions. Exposure to daylight may occur, for example, for the uppermost precursor of a stack of precursors loaded in a platesetter. It has been observed that when the platemaking process is resumed, for example after a couple of days, the lithographic properties of such an uppermost precursor of a stack of precursors may have changed compared with those of the other precursors of the stack. This may necessitate a remake of a printing plate resulting in productivity loss at the customer site.
EP 2072570 discloses lithographic printing plate precursors comprising thermoplastic polymer particles and infrared dyes having a specific central structural element characterized by an improved storage stability.
Adding stabilizers is another strategy to optimize the stability of infrared dyes towards oxidative and/or light induced deterioration. Numerous classes of stabilizers have been reported in the literature on lightfastness and oxidation stability of dyes. UV absorbers, hindered amine light stabilizers, i.e. HALS-compounds, and different types of antioxidants are generally used. Typical UV-absorbers are 2-hydroxyl benzophenones, 2-hydroxyphenyl benzotriazoles, 2-hydroxyphenyl triazine, bis oxalyl amide derivatives and cinnamate derivatives. Typical HALS-compounds are 2,2,6,6-tetramethyl piperidine derivatives and the infrared radiation absorbing corresponding piperinyloxy radicals. Typical antioxidants can be selected from sterically hindered phenols, tocopheroles, catechol derivatives, hydroquinone derivatives, gallic acid esters, hydrazides, hydroxyl amines, hydroxamic acids, phosphites, thioethers and reductones such as ascorbic acid and its derivatives. EP-A 1 910 082 discloses the use of specific carboxylic acids to improve the daylight stability of printing plate precursors comprising specific infrared dyes and hydrophobic thermoplastic particles. Phenyl glycine is disclosed as being particularly preferred.
EP-As 1 974 911 and 1 974 912 both disclose the use of ascorbic or erythorbic acid derivatives in lithographic printing plate precursors comprising an infrared dye and hydrophobic thermoplastic particles.
However, adding such stabilizers to a lithographic printing plate precursor may lead to an unacceptable worsening of the lithographic properties, especially the sensitivity, of the printing plate precursors. Therefore, there is still a need for more optimal stabilizers to improve the storage and light stability of heat sensitive lithographic printing plate precursors without adversely affecting their other lithographic properties.